It is often desirable in electrohydraulic servo systems to control both system output flow and pressure in separate flow-control and pressure-control modes of operation. For example, in electrohydraulic control of an injection molding machine, it is desirable initially to operate the machine injection cylinder in a flow-control mode of operation for controlling flow of fluent plastic into the mold cavity, followed by a pressure-control mode of operation for maintaining desired pressure profile after the cavity is full. In electrohydraulic systems which employ electronic feedback of system pressure for closed-loop pressure control, problems arise because servo loop gains vary with cavity volume, and because effective cavity volume often varies either by operator intent or because of fluid leaks in the system. Total compression volume, including molten plastic in the mold cavity, may vary by a 12:1 ratio depending upon size and type of mold. However, a volume change as low as 10% to 20% requires adjustment of servo loop gains to maintain desired performance. In the past, loop gains have been adjusted manually (if at all) on an essentially empirical (trial and error) basis.
It is therefore a general object of the present invention to provide an electrohydraulic servo control system of the described character which includes facility for closedloop electronic control in a pressure control mode of operation in which loop gains are automatically adaptively adjusted during operation as a function of cavity volume. Another object of the invention is to provide an electrohydraulic servo system which includes facility for improved adaptive control of hydraulic fluid flow in separate flow-control and pressure-control modes of operation.